Beyond Kinetics: Thermodynamic Insights into Sulfur Conversion Mechanism in Metal–Sulfur Batteries

Abstract

Understanding the sulfur redox mechanism is essential for advancing lithium–sulfur (Li–S) batteries. Conventional mechanistic studies primarily rely on kinetic analyses that monitor intermediate species during electrochemical cycling, offering limited thermodynamic insight. Here, we introduce a thermodynamic approach using collision-induced dissociation (CID) mass spectrometry to probe the intrinsic stability and S–S bond dissociation pathways of polysulfide species formed via electrospray ionization. A series of polysulfide anions (including S3–, S4–, LiS3–, LiS4–, LiS5–, LiS6–, LiS7–, LiS8–, LiS9–, LiS10–, and LiS11–) were identified and systematically examined. The CID spectra revealed chain-length-dependent S–S bond strengths and dissociation preferences: long-chain polysulfides favored medium-chain fragment formation, medium-chain species primarily lost S2, and short-chain polysulfides generated S2– through LiS or LiS2 loss. These findings provide direct thermodynamic evidence supporting a modified four-stage discharge mechanism for Li–S batteries. Compared to the conventional three-stage discharge mechanism, the proposed four-stage discharge mechanism differs in three key aspects: (1) the sloping region connecting the first and second discharge plateaus is assigned to the reduction of long-chain lithium polysulfides (Li2SL, L ≥ 11) to soluble medium-chain lithium polysulfides (Li2Sm,10 ≥ m ≥ 5); (2) the second discharge plateau is attributed to the reduction of medium-chain lithium polysulfides (Li2Sm,10 ≥ m ≥ 5) to soluble short-chain lithium polysulfides (Li2Ss, s = 3, 4) and insoluble Li2S2; (3) the reduction of short-chain lithium polysulfides (Li2Ss, s = 3, 4) to insoluble Li2S and Li2S2 only happens below 2 V. Comparison of lithium and sodium polysulfides further demonstrates the method's applicability to other metal–sulfur systems. This thermodynamic mass spectrometric approach offers an experimental framework for elucidating redox mechanisms in sulfur-based batteries and can be extended to study charge-transfer processes in future ion-trap or electron-transfer dissociation systems.